This invention relates to leak detection systems and methods of the type employed with underground tanks for storing highly vaporous liquids such as gasoline at gasoline service stations. Such storage tanks, which may consist of, for example, a twenty-six foot long cylindrical tank having a diameter of eight feet, and holding perhaps ten thousand gallons of gasoline, are conventionally buried in the earth below the frost line, and it is necessary to periodically test such storage tanks for leaks. Normally, unaffiliated private equipment operators with portable equipment are employed by station operators to perform leak tests. The methods of testing have varied widely. Typical commercial methods of testing today do so by measuring the changes in volume of the liquid contained in the tank over a period of time by various methods and typically also it is necessary to compensate for temperature change, because changes in temperature effect changes in volume. Because the temperature of the various strata of liquid in a tank differs, any temperature reading taken at one location in a tank is not accurate for all of the liquid in the tank and thus taking an average temperature and correcting for temperature change creates considerable problems for the testing equipment, and may not always provide the accuracy which is deemed necessary for safety. The methods and equipment described in some of the following prior art patents, which I incorporate herein by reference, are exemplary of testing methods which have been proposed:
______________________________________ 2,853,874 Mennesson 2,912,852 Trinneer 3,580,055 White 4,281,534 Hansel 4,462,249 Adams 4,386,525 Mooney 4,474,054 Ainlay 4,571,987 Horner 4,618,268 Horner 4,646,560 Maresca 4,649,739 Horner 4,885,931 Horner ______________________________________
Previously, it has been possible to test storage tanks for heavier hydrocarbon liquids such as diesel fuel by applying a vacuum or negative pressure to the tank ullage, isolating the tank, and then simply measuring the decay of the negative pressure over a time period. The Trinneer Pat. No. 2,912,852 discloses such a tank testing device. This method has, to my knowledge, not been sufficiently feasible, in my view; with highly vaporous, volatile products such as gasoline which, when placed under a sufficient negative pressure for measuring, react to the negative pressure by vaporizing at a high rate which decreases the negative pressure so rapidly that sensitive tests cannot be performed.